In STEM project group teams, men speak for more time (Meadows and Sekaquaptewa, in: Proceedings of ASEE annual conference, 2011) and engage in more active technical participation than women, which can have negative long-term consequences (Cheryan et al. in Psychol Bull 143:1-35, 2017; Lord et al. in IEEE Trans Educ 54(4):610-618, 2011). In the current study, we tested the effects of a brief counter-stereotypic video intervention on gender gaps in verbal participation on mixed-gender teams of STEM students (N = 143). Participants viewed either a control video of an engineering student team behaving in a gender stereotype-consistent way (men talked longer and presented more technical information than women) in a group presentation and group interview, or a gender counter-stereotypic intervention version (roles reversed) prior to engaging in their own STEM group project task in a laboratory setting. Analysis of video footage of the groups showed that men spoke longer than women in the control condition, but men and women spoke for equal time in the intervention condition. This result was corroborated by participants' self-report of their verbal participation in their group.